The present invention relates to a liquid crystal electro-optical device having excellent electric properties and contrast, and which is capable of realizing a bright and uniform display over the entire image plane.
According to the basic principle for the operation and the display of a liquid crystal electro-optical device, in general, a liquid crystal electro-optical device has such a constitution that an organic material, more specifically, a liquid crystal material, is interposed between a pair of substrates, and the light passing through the liquid crystal material is modulated by changing the intensity of the electric field which is generated by the electrode formed between the pair of substrates. As a result of the modulation, the change is recognized as the change in the quantity of light.
Accordingly, if a specified electric signal is supplied to the electrode, the electric signal can be displayed as a visually observable state. Thus, a desired image can be formed by combining a plurality of electrodes and by then applying an electric signal corresponding to the image data.
The optical modulation in a conventional liquid crystal electro-optical device is realized by applying the electric field perpendicular to the substrate, and then changing the intensity of the electric field to thereby change the alignment direction of the rod-like liquid crystal molecules in the direction parallel to the substrate or in the direction perpendicular to the substrate. In this case, in general, a polarizer sheet must be incorporated into the device to obtain a linearly polarized light from the incident light, because the light is modified by using the optical anisotropy of the liquid crystal material, i.e., one of the characteristics of a liquid crystal material.
However, in case of a liquid crystal electro-optical device based on the operation principle above, although a normal display state is realized when the display plane is viewed from the direction perpendicular thereto, the display becomes dark and unclear when the display plane is viewed from a direction oblique thereto; in case of a color display, moreover, a decoloring phenomenon occurs.
By considering the relation between the output light from the liquid crystal electro-optical device and the alignment direction of liquid crystal molecules, the phenomenon above can be explained as follows.
In case of employing a constitution as such that the liquid crystal molecules are aligned in a direction perpendicular to the substrate, the molecules are aligned in a direction as such that the major axes thereof are arranged in parallel to each other in displaying an image, but the observed light is a light from the perpendicular plane of randomly positioned liquid crystal molecules.
On comparing the case of viewing the display from the direction perpendicular to the substrate and the case of viewing it from a direction slightly deviated from the direction perpendicular to the substrate in the constitution above, the display as viewed from the latter view point signifies that the display is viewed from a viewpoint slightly tilted with respect to the major axes of the liquid crystal molecules. This indicates that the observation area of the output light greatly differs depending on the direction of viewing the display plane.
Accordingly, the degree of degradation of visual field characteristics of an observer increases with increasing deviation from the direction perpendicular to the display plane.
Furthermore, there is another problem in case of a liquid crystal electro-optical device above. That is, the liquid crystal material is aligned in a specified direction by generally applying a certain alignment treatment to the substrate. However, because a strong alignment force functions in the vicinity of the substrate, the liquid crystal molecules in the vicinity of the substrate maintain the aligned state even when an electric field is applied thereto, or, the change in the aligned state appears far smaller as compared with the liquid crystal molecules positioned in the central portion of the device. Accordingly, the light is scattered in the vicinity of the substrate as to influence the display.
As means for solving the problem above, there is proposed to change the optical characteristics by an operation mode differing from that of a conventional liquid crystal electro-optical device; i.e., the liquid crystal molecules are rotated only in a direction parallel to the substrate. The details are disclosed in JP-B-Sho 63-21907 (the term “JP-B-” as referred herein signifies “an examined published Japanese patent application”). This operation mode is referred herein to as “IPS mode”.
A liquid crystal electro-optical device operated by IPS mode is characterized in that the opposing electrode for driving the liquid crystal material, which is provided to the opposing substrate side is disposed on the substrate side on which the pixel electrode is provided. That is, a pixel electrode and an opposing electrode are provided on one of the pair of substrates that are provided faced to each other.
Thus, an electric field is formed between the pair of electrodes formed on a single substrate. The electric field comprises the principal component thereof in the direction parallel to the substrate and the liquid crystal layer. The liquid crystal molecules are thus rotated inside a plane parallel to the substrate by this electric field. Accordingly, the liquid crystal material, which is an optically uniaxial medium, changes the optical axis by the applied electric field, and the state of light transmitted through the liquid crystal layer is changed by the birefringence effect as to make a display possible.
Thus, as described above, the liquid crystal molecules are never aligned perpendicular to the substrate during the operation under an operation in IPS mode. Accordingly, the problem of visual angle attributed to the perpendicular alignment of the liquid crystal molecules in the operation process can be solved.
In the operation under IPS mode, a switching element such as a thin film transistor is connected to the pixel electrode to realize active matrix drive.
However, a first disadvantage of this constitution (operation in IPS mode) is the liquid crystal alignment in the dark display state, i.e., the OFF state of electric field. In general, it is preferred that the liquid crystal is uniformly arranged over the entire substrate in the state of turning OFF the electric field.
In practice, however, alignment defects form depending on the state of rubbing, and a uniform alignment over the entire substrate cannot be obtained. Accordingly, a uniform black display cannot be obtained in the practical display of black color. This is a problem which cannot be neglected in making substrates large-sized.
A second disadvantage of the operation in IPS mode is that the intensity distribution of the electric field is non-uniform. In the case of IPS-mode operation in which an electric field is applied in the direction parallel to the substrate to realize a display, the electrode for forming the electric field is provided only on one side of the substrates. The electric field to be applied to the liquid crystal molecules becomes weaker with approaching the opposed substrate, i.e. increasing a distance from the substrate having thereon the electrode.
Accordingly, a uniformity on the entire display cannot be obtained due to the non-uniformity in the rise time.
A third disadvantage of the operation in IPS mode is the low opening ratio (low aperture ratio). In the case of operation in IPS mode, the liquid crystal is controlled by the electric field formed between the pair of electrodes formed on the same plane. Thus, liquid crystal present on the upper side and in the vicinity thereof cannot be controlled. This surely lowers the aperture ratio by the area corresponding to the electrode.
In particular, because bright and dark states in case of the operation in IPS mode are displayed by using the polarization of light, a polarizer sheet is indispensable. The two polarizer sheets that are incorporated in the device further lower the optical transmittance.
A dispersion type liquid crystal electro-optical device is known as another liquid crystal electro-optical device which overcomes the disadvantage of low optical transmittance attributed to the presence of polarizer sheets. A dispersion type liquid crystal electro-optical device is characterized in that it requires no polarizer sheets nor molecular alignment.
The constitution of a dispersion type liquid crystal comprises granular or sponge-like nematic, cholesteric, or smectic liquid crystal sustained in a light-transmitting solid phase polymer.
A liquid crystal electro-optical device of this type can be fabricated by dispersing the liquid crystal inside the polymer by encapsulating the liquid crystal, and then forming the polymer as a film or as a thin film on a substrate. Proposed as substances for use in the encapsulation include gelatine, gum arabic, poly(vinyl alcohol), and the like.
Examples of a film or a thin film of a polymer material containing the encapsulated liquid crystal dispersed therein include the following other than those described above. For instance, mentioned are:    (1) a material comprising liquid crystal material dispersed in an epoxy resin;    (2) a material utilizing phase separation of a liquid crystal and a photocurable substance; and    (3) a material comprising a three-dimensionally connected polymer material impregnated with liquid crystal;    In the present specification, liquid crystal electro-optical devices represented by those described above are referred to collectively as “dispersion type liquid crystal electro-optical device”.
The operating principle of the above-described dispersion type liquid crystal electro-optical device is described below. In a dispersion type liquid crystal electro-optical device, the liquid crystal is randomly oriented without being aligned to a particular direction in case no electric field is applied thereto (state with no electric field). In such a state, the light is scattered because the refractive index of the liquid crystal does not match with that of the polymer surrounding the liquid crystal. Thus, the transmission of light is obstructed as to realize a white opaque state corresponding to the dark state of the liquid crystal electro-optical device.
If an electric field is applied in the perpendicular direction at this instance, the major axes of the liquid crystal molecules align perpendicular to the substrate. Thus, if the refractive index in the direction of major axes of the liquid crystal is adjusted as such that it may match with the refractive index of the polymer resin, a state with uniform refractive index can be realized to prevent light scattering from occurring. In this state, light permeates the liquid crystal layer as to realize the bright state of the liquid crystal electro-optical device.
Thus, light can be utilized effectively in this case because the electro-optical effect is realized without using any polarizer sheets.
However, in a practical dispersion type liquid crystal electro-optical device, the opacity depends on the degree of light scattering under the state of no applied electric field. Thus, there is a problem that a display with high contrast cannot be realized. Although there is a problem of lightness, a liquid crystal electro-optical device using a polarizer sheet still claims superiority.
Another problem in the dispersion type liquid crystal electro-optical device is that, in the bright state, the liquid crystal molecules align themselves in such a manner that the major axes thereof become perpendicular to the substrate plane. Similarly, in a dispersion type liquid crystal electro-optical device again, the problem of visual angle as described above also remains.
As described above, a liquid crystal electro-optical device operating in IPS mode is characterized in that it has a wide viewing angle. However, it has disadvantages in that it has difficulty in increasing the opening ratio (aperture ratio), that the electric field is non-uniform, and that the display plane becomes dark by the use of a polarizer sheet, etc. To increase the opening ratio (aperture ratio), it is required to further introduce the technology of advanced lithography and to improve the liquid crystal material and the like for sustaining the image data.
In contrast to the liquid crystal electro-optical device operating in IPS mode above, a dispersion type liquid crystal electro-optical device is characterized in that it can effectively utilize light because it can output the incident light as it is without using any polarizer sheets. However, on the other hand, as described above, it has difficulty in realizing an image with high contrast, and, similar to conventional liquid crystal electro-optical devices, it has a disadvantage in that it greatly depends on the visual angle.